The present invention relates to an electronic ballast and a method for providing arc straightening to a discharge lamp. The invention may be particularly useful in connection with high-intensity discharge (HID) lamps powered from a universal input and/or a 277 V AC line.
In the field of ballasts for HID lamps, it is known that operating at relatively high frequencies can produce any number of advantages including decreases in the size and weight of the ballast, as well as increases in lamp efficacy. A significant problem of high frequency ballasts is the acoustic resonance often introduced by the use of such a system, and the arc instabilities that can result therefrom.
The prior art illustrates a number of approaches to overcoming these problems. Among these, U.S. Pat. No. 5,623,187 to Caldiera et al. describes one known approach. As described in this reference, arc instabilities are accompanied by deformations in the arc which change the arc""s length, which in turn is known to vary the conductance or impedance of the operating lamp. The Caldiera et al. reference also teaches the necessity of adjusting the modulation frequency of the signals sent to the lamp in order to minimize the effects of acoustic resonance.
It is an object of the present invention to provide a universal input voltage electronic ballast to reliably regulate lamp power.
It is a further object of the invention to provide arc straightening for mercury-free HID lamps to improve the luminous efficiency of such lamps.
It is a further object of the present invention to provide a microprocessor control circuit arrangement for programmable start of a universal voltage electronic ballast having an active power factor corrector and a DC-AC converter.
It is another object of the present invention to provide a microprocessor control circuit arrangement for programmable start of a universal voltage ballast having an inrush current limit circuit, an active power factor corrector and a DC-AC inverter.
It is another object of the present invention to provide a microprocessor control circuit arrangement for instantaneous power regulation and programmable start of universal voltage ballast having an inrush current limit circuit, an active power factor corrector, and a DC-AC inverter.
The present invention includes an improved method and apparatus for controlling the ballast for an HID lamp so that arc straightening may be obtained through a simplified approach based on power consumption.
The method provides for controlling a lamp ballast which includes a DC-DC converter with first and second switches which alternately connect an input side of the DC-DC converter to a high-voltage DC power source and ground, so as to drive a power transformer, with an output of the power transformer being connected to a DC-AC inverter which in turn drives the lamp.
The method includes the steps of alternately closing the first and second switches at a frequency which is swept repeatedly between predetermined minimum and maximum frequencies, determining a present level of power consumption by the lamp, and controlling the first and second switches so that a ratio of the time during which either of the switches is closed compared to a length of the cycle of opening and closing such switches is adjusted based on the determined present level of power consumption.
In order to determine the present level of power consumption, it is possible to sense the lamp voltage and the lamp current and multiply the two values. The results of this multiplication can be used to generate a power control signal whose level reflects the calculated power consumption. The power control signal can be provided to a pulse width modulator (PWM) which in turn controls a duty cycle of generated pulse width modulated signals that the PWM uses to drive the switches. The repetition rate at which the PWM switches the pulse width modulated signals may be determined by a variable frequency signal which is repeatedly swept between a predetermined minimum and predetermined maximum frequency. The PWM may receive as a further input a signal whose level is indicative of sensed primary current of the ballast.
The PWM therefore many receive as inputs the variable frequency signal, a power control signal indicative of power consumed by the lamp, and a sensed primary current signal. The nature of operation of the PWM is such that an increase in the level of either of the power control signal and the sensed primary current signal will tend to decrease the duty cycle of the pulse width modulated outputs of the PWM used to drive the switches. Similarly, an increase in the frequency of the swept frequency signal will tend to decrease the duty cycles.
The PWM may be designed so that the received power control signal is internally compared to a power reference signal to produce an error signal, with the error signal being compared to the sensed primary current signal to ultimately control be duty cycles of the PWM outputs used to control the switches. The switches may be power MOSFETs or IGBT switches, with the pulse width modulated outputs of the PWM being used to drive a gate drive transformer. The gate drive transformer in turn produces as outputs gate drive signals connected to the gates of the power MOSFETs.